1. Field of the Invention
The present invention relates to the technology of an optical storage medium, and more particularly to a method for adjusting a tilt (or tilt angle) of an optical pick-up head.
2. Related Art
Typically, an optical pick-up head of an optical recording and reading device needs an actuator for adjusting an object lens so that this device can read information from the disk or record the information onto the disk at correct locations. This actuator can precisely move the optical pick-up head to the desired location and tilt the optical pick-up head by a correct angle. However, when the optical disk to be read or recorded is curved or the tilt angle of the optical pick-up head is incorrect, the read or recorded information deteriorates. In order to compensate for the information deterioration, the relative tilt angle between the optical pick-up head and the optical disk has to be measured and the tilt angle of the optical pick-up head has to be timely adjusted.
FIG. 1 shows the architecture of a conventional tilt adjusting device. Referring to FIG. 1, this architecture includes an optical pick-up head 10, a tilt angle detector and an object lens 16. The optical pick-up head 10 is pivotally mounted on a rotating shaft 18 and is controlled to rotate by the actuator (not shown in this drawing). The tilt angle detector has a light emitter 12 for emitting light rays, and light receivers 14a and 14b for receiving reflected light rays to detect the tilt angle, according to which the actuator may be controlled. This method has to utilize the tilt angle detector to detect the relative tilt angle between the optical pick-up head and the optical disk to serve as the basis for adjusting the tilt angle of the optical pick-up head.
FIG. 2 shows a method of adjusting a tilt angle of an optical pick-up head without the need of a tilt angle detector. Referring to FIG. 2, a tilt control module 20 includes a reproduction signal generating unit 22, a jitter detector 23, a tilt control unit 24, a tilt actuator 25 and a memory 26. The reproduction signal generating unit 22 converts a radio frequency (RF) signal, generated by the optical pick-up head, into an eight-to-fourteen modulation (EFM) signal, and utilizes a slicer to slice the signal into 0 and 1 signals. The jitter detector 23 receives the EFM signal and detects a jitter amount of the EFM signal. The typical jitter amount is obtained by calculating an extent of back and forth wandering of the EFM signal. The tilt control unit 24 outputs different tilt control values corresponding to different tilt angles of the optical pick-up head, records the corresponding jitter amounts, and screens the tilt control value, corresponding to a minimum jitter amount or a jitter amount smaller than a jitter threshold value, as the tilt control value for the track. The tilt actuator 25 outputs a driving signal to an optical module 21 according to the tilt control value of the tilt control unit 24 to change the tilt angle of the optical pick-up head. The tilt control value may be a digital signal stored in the memory 26, and a digital-to-analog converter (DAC) converts the tilt control value and then outputs the converted tilt control value to the tilt actuator 25. The tilt actuator 25 converts a signal, such as a voltage signal ranging from 0 to 4V and corresponding to the tilt control value, into the driving signal for rotating the optical pick-up head.
Although this method can adjust the tilt angle of the optical pick-up head to a better location without the need of a displacement detector, only the optical disk with the written data can be used to adjust the tilt angle of the optical pick-up head according to the jitter amount of the EFM signal. A blank optical disk cannot be adopted to adjust the tilt control value because the EFM signal cannot be obtained therefrom.
FIG. 3 is a schematic illustration showing a method of calibrating a tilt of an optical pick-up head disclosed in U.S. Patent Publication No. 2005/0117473. As shown in FIG. 3, it is assumed that a surface of an optical disk 31 is an inclined plane in this method. That is, the portion of the optical disk 31 between locations P1 and P2 is regarded as a straight line. After an optical pick-up head 32 is focus-controlled, a fixed distance is kept between the optical pick-up head 32 and the optical disk 31. That is, after the focus control mechanism is enabled, the optical drive outputs a focus control signal (FCS) for controlling up and down movements of a lens of the optical pick-up head according to a focus error (FE) signal such that a fixed distance is kept between the lens of the optical pick-up head and the optical disk 31. So, as shown in FIG. 3, when the optical pick-up head 32 is located at the locations P1 and P2, the relative distances between the lens of the optical pick-up head and the optical disk 31 are constant. Therefore, the tilt angle can be calculated only according to the distance λ between the locations P1 and P2 and the corresponding height difference (lens motion distance β) between the locations P1 and P2. The tilt angle is calculated according to the following equation:
  θ  =            tan              -        1              ⁡          (              β        λ            )      
In fact, however, the surface of the optical disk is not an inclined plane but approximates a curved surface. So, the assumptions mentioned hereinabove only can be made when the distance between the locations P1 and P2 is very small and when the optical disk is not seriously curved. If the optical disk is seriously curved, various locations on the optical disk have to be measured so that the above-mentioned method can precisely calibrate the tilt angle of the optical pick-up head. However, the spent time becomes longer with the increase of the number of the locations. In addition, the above-mentioned method is to approach a curve with straight lines.